Frequency discriminator



Oct- 2, 1951 B. J. VAN HARDENBERG 2,570,013

FREQUENCY DISCRIMINATOR Filed June 17, 1949 2 Sheets-Sheet 1 Il IIIIIIIIIIII. T IIII ILI. I IILPII AGI/VT 3 1@ 'III 1|III 9 x.. .Il IIfI r M I I ,.l .5 II IIT I I I I I I I I L.

| I I I \I Oct' 2, 195l B. J. VAN HARDENBERG 2,570,013

FREQUENCY DISCRIMINATOR Filed June 17, 1949 2 Sheets-Sheet 2 Patented Oct. 2, 1951 2,570,013 FREQUENCY DISCRIMINATOR Bonifacius Johannes van Hardenberg, Eindhoven, Netherlands, assigner to Hartford `National pany, Hartford, Conn., as

Bank and Trust Com trustee Application June 17, 1949, Serial No. 99,752 In the Netherlands July 16, 1948 7 Claims.

In practice there is a need for a device which supplies an indicating voltage whose polarity varies according to the sign of a frequency difference between any alternating voltage and a pulse-shaped voltage or a higher harmonic thereof. Such a comparison frequency discriminator may be used with particular advantage for frequency measurement; or frequency control.

This invention has for its object to provide simple means for producing such an indicating voltage, when thesaid alternating voltage has a sawtooth waveform. As will be set out hereinafter, the device according to the invention may also be employed, if the said alternating voltage is, for example, sinusoidal, the sine voltage being ilrst converted into a sawtooth voltage, as may be effected in a variety of ways known per se.

Such a frequency discriminator according to the present invention is characterized by the series combination of means for producing a sawtooth beat voltage at the difference frequency to be indicated and a differentiating network, from the output circuit of which the indication voltage is taken.

The said means for producing the sawtooth beat voltage may be constituted by devices known per se from the communication art for pulsephase demodulation relatively to a sawtooth voltage and of the kind described, for example, in U. S. Patent No. 2,471,168, issued May 24, 1949. Thus, for example, use may be made of a pulse modulator for causing the sawtooth voltage to amplitude-modulate the pulses, the modulation being followed by a low-pass illter for the amplitude-modulated pulses, the low-pass illter preferably forming part of a peak detector.

According to a further feature of the invention a rectified indicating voltage, whose polarity varies with the sign of the frequency difference, is obtained from the output of a push-pull peak detector connected to the differentiating network,

In order thatI the invention may be more clearly understood and readily carried into effect, one example will now be described with reference to the accompanying drawing, of which Fig. 1 shows the block schematic diagram of a device according to the invention, the operation ci which is described more fully with reference to the voltage-time diagrams shown in Figs. 2 t 5; l

Fig. 6 is a detail circuit diagram of the device and Fig. 7 shows a voltage-time diagram to explain the manner of converting `a sine voltage into a 2 sawtooth voltage, according to the circuit of Fig..6.

Referring to Fig. 1, an oscillator I produces a sine voltage, the divergence of the frequencyof which voltage with respect to the frequency of a component of the spectrum of a pulse-like voltage produced by an oscillator 9 is required to be indicated with its correct polarity. The sine voltage of oscillator I is used for synchronising a sawtooth generator 2, the output voltage of which is fed as a modulating voltage, to a pulse modulator 3 for the amplitude-modulation of the pulses taken from the pulse generator 9.`

In Figs.` 2a and 2b are shown the sawtooth voltage and the pulse-like voltage which are fed to the pulse modulator 3, it being assumed for the sake of simplicity that the fundamental frequencies of the two voltages are nearly equal.

In Fig. 2c are shown the amplitude-modulated pulses taken from the output circuit of the pulse modulator 3. It is evident therefrom that the envelope of the modulated pulses exhibits a sawtooth variation, the frequency of this sawtooth beat voltage corresponding to the difference of the 4fundamental frequencies of the sawtooth and pulse-like voltages fed to the modulatorV 3. The pulse repetition frequency of the pulses shown in Fig. 2b is slightly smaller than the fundamental frequency of the sawtooth voltage shown in Fig. 2a, so that the envelope of the modulated pulses, as shown in Fig. 2c, is similar in shape to the sawtooth voltage shown in Fig. 2a.

Figs. 3a, 3b and 3c show the relationships produced if the pulse repetition frequency, or a higher harmonic thereof, is slightly higher than the frequency of the sawtooth input voltage to the mixing stage 3. The pulse-like voltage shown in Fig. 2b has a repetition frequency such that double its value slightly exceeds the frequency of the sawtooth voltage shown in Fig. 2a.

It should be emphasised that the foregoing and the following also applies to higher harmonics and, in the event of a sufliciently short pulse even to 4very high harmonics, for example the 250th harmonic of the pulse repetition frequency.

By mixing the voltages shown in Figs. 3a and 3b, amplitude-modulated pulses are produced, the envelope of which shows a sawtooth variation the frequency of which corresponds to the frequency `difference between the sawtooth voltage shown in Fig. 3a and the rst harmonic of the pulselike voltage shown in Fig. 3b. However, the shape of the sawtooth beat voltage shown in Fig. 3c diverges from that of the sawtooth beat voltage shown in Fig. 2c, since the sign of the frequency diierence of the voltages fed to the mixing stage 3 is now opposite. With the frequency ratios shown in Fig. 3 a sawtooth beat voltage is produced, which is a mirror image, with respect to the ordinate, of vthat shown in Fig. 2c. ,y

Figs. 4a and 5a show the sawtooth beat voitages corresponding to Figs. 2c and 3c respectively to a reduced time scale, it being assumed that, by smoothing with the use of a low-pass lter 4 following the mixing stage 3, the pulse repeti= tion frequency and higher harmonics thereofare eliminated from the mixed voltage.

By differentiation f the beat voltages ShW'II in Figs. 4a and 5clr with the use of a differentiaing network 5, ,there are producedA in the one case negative pulses and in the other case positive pulses, as shown in Figs. 4o and 5b, said pulses indicating by their polarity the sign of the frquency difference required to be known. l

. As. shown in Fig. 1., this indicating voltageV is fed to an indicator 6 which is operative tov indi;- cat 'the polarity of the pulses. Qbviously. the indicating Voltage obtainedlnay be utili-Zedfor example subsequent toreotiication with the use of a detector 1, for-controlling the-value 'of' a con-'- trollable reactance 8,- which is coupled to the frequency-determining circuit oi the oscillator I for automatic' correction of the oscillator frequency, accordingY to the pulse repetition fre-'- quenc'y or a higher harmonic' thereof.

' 'InFigure'Syr'ef'erence numerals Il and 9 sirnila-rlydesignate an oscillator for a sine voltage and anroscillator for a pulse-like voltage respectively; VThe sine voltage from oscillator I is fed to aY converter "2 to produce a Sawtooth voltage, said converter comprising a transformer I= and a ha'liwave rectier I It which is connected there= to and which includes an output resistance I3 shunt-ed by a smoothing condenser I2. 'Y

The time constant-ofthe smoothing filter I2,

I is chosen to be smalier than one period of the sine* voltage; supplied, so that, in the manner shown in Fig; 7, the sawtooth Voltage b, shown in dotted linefsrin Fig; '7, is produced from the 'sine voltage a;

The sawtooth voltage taken from the converter 2 vand thepulse-liieevpltage of the oscillator s are to a peritode Ie, used as a mixing tube, which i's normally cutV off by a` highly negative gridI bias voltage applied to the rst control=grid; This bias voltage is taken from a'potentionleter comprising a resistance I3 connected'to the posi; tive'terminal I5 of" an anode voltage source, not shown in the diagram and cathcde'resistances I'I 'and' I8'. The pentodeA It isperiodically gated by the pulses supplied thereto, eachitizne passing an anode current which varies with the instan# taneous value of the sawtooth voltage 'ferito `the second control-grid; V'Thus amplitude-modulated pulses, as shown in Figs. 2c andc, are developed across the anode resistance IS'olfthe mixing-tube, The" output voltage of the ini'x'ingrstageis' fed throughV a low-*pass iil'ter fi', having a cuteoff free quencycorresponding, forY example, to half thev frequency interval between successive components of Y the speetrumgof thepulse-like voltage and ornliising Series rsistances 2l] ad-by-p'asi O-lde' isers 2i, to` a'diiferentiating network-5Com# pl jg a seriesconclenser 23 'and 'anoutpiit resistiranc 2S. Set up across this output resistance 23 antiwar-ying withV the sign of the kiresiliency differencelbet'ween the cor'npared voltages 're nega# .tive'orpositive Voltagulss, alshw '-iivFigsa 4b and 5b. 'Y AS--ShWliI1Fig-. Gy this Voltagfe vrhisrectied indicating voltage may be fed to a direct current instrument 28 having a central zero position, which thus provides a visual indication of the sign of the frequency difference.

In the embodiment shown in Fig. 6, the lowpass filter 4 produces a material damping of the beatvo-ltage fed thereto. This disadvantage may be oh-viatedby including a peak detector in the anode circuit of the mixing tube I4, this detector being*V connected, for example, `by means of a V coupling condenser, to the anode circuit of the mixing tube.

is obvious that the differentiating network for the sawtooth beat voltage may be constituted notonly by the resistancezcapacitance network shown but also by other differentiating networks", for'. example, a diierentiating' network conipris inga resistance andan 'inductance Y Finally, it Sliouldrbe noted` that in the embodi-Y rnent shown in Fig. 6 the olii'erentiatingY network 'aiid 'the puShedll-p'e'ak detector 24' may, if dsired, be shunted a high ohmic resistance, as

iarasitiso'f interest, in the case of synchronism of the compared frequencies, to produce an output voltage 'Varying with the phase relation of the compared' voltage, as may be particularly imil. Apparatus for producing an outplt voltage whose polarity depends on the-sign ofv the freqtiency difference between a saw tooth voltage and a component in the harmonic spectrur'n'ofa periodic pulsatory' voltage Acomprising meansl to combine saidr sawY tooth voltageY and said pulsa'e toryvoltage to produce a saw tooth Wave whose frequency corresponds `to'tll'e Ydifference in fre'- du'er'lc'y between said saw tooth voltage andv said component,` a differentiating. network, Y"and inea'ns to applysalo. saw tooth wave to said network to produce 'said output voltage. Y

2. Apparatus for producing aneutputvoltg .vnoseipolarity depend on the sign of the frequei/roy.y difference pf' een a 'saw tooth voltage anda foonipoei'ittiri 'th'elharrhonic 'spectrum of periodic pulsatory voltage comprising a .pulse modulator to combinesaidsaw'tcoth voltage with pulsatory voltageV to produce resultantp'ulses which are` amplitude' modulated by a VsawV tooth wave whose V4freq'uenc'y corresponds Yto the difference in frequency between said saw tooth Volt#- age andsaid component, a low-passlter coupled to 'theoutput'oi saidpulse-inodulator orextracting: solelyY said isaw tooth wave therefrom, Ya difierentiatinginetwork, andmeans.toV apply `said saw tooth wave to said network to produce said output voltage. d y n n v, l 34. Apparatusuas set forth in claim v2, further includinga peak detector coupled to the output of Vsaid Vdiierentiating network to produce an indicating voltage, and an indicator coupled t theoutputrof said peak detector. 'Y

4. Ina control system for a sinusoidal oscillat/or; 'the combination comprising means to convert the oscillations from said oscillator in to a saw tooth voltage, al periodic pulsatory Voltage source, Vm'teansto 'combine said saw 'tooth voltage and said pulsatory voltage to produce a saw tooth wave whose-@frequency gcorrespondslto the dinerage and a component in the harmonic spectrum of said pulsatory voltage, a differentiating network, and means to apply said saw tooth wave to said network to produce an output voltage whose polarity depends on the sign of the frequency difference between said saw tooth Voltage and said component.

5. In a control system for a sinusoidal oscillator, the combination comprising means to convert the oscillations from said oscillator into a saw tooth Voltage, a periodic pulsatory voltage source, means to combine said saw tooth voltage and said pulsatory voltage to produce a saw tooth wave Whose frequency corresponds to the difference in frequency between said saw tooth voltage and a component in the harmonic spectrum of said pulsatory voltage, a differentiating network, means to apply said saw tooth wave to said network to produce an output voltage Whose polarity depends on the sign of the frequency difference between said saw tooth voltage and said component, means to detect said output voltage to produce a control voltage, and a frequency control devce coupled to said oscillator and responsive to said control voltage for effecting synchronism between the frequency of said oscillator and said component.

6. In a control system for a sinusoidal oscillator the combination comprising means to convert the oscillations from said oscillator into a saw tooth voltage, a periodic pulsatory voltage source, a pulse modulator to combine said saw tooth voltage and said pulsatory voltage to produce resultant pulses amplitude modulated by a saw tooth wave Whose frequency corresponds to the diierence in frequency between said saw tooth voltage and a component in the harmonic spectrum of said pulsatory voltage, :a 10W-pass iilter coupled to said modulator to extract solely said saw tooth wave therefrom, a differentiating network, means to apply said saw tooth Wave to said network to produce an output voltage whose polarity depends on the sign of the frequency difference between said saw tooth voltage and said component, a push-pull peak detector coupled to the output of said network to produce an indicating voltage, and an indicating device coupled to the output of said detector.

7. A system, as set forth in claim 6, wherein said low-pass lter has a cut-off frequency corresponding to one-half the periodicity of said component.

BONIFACIUS JOHANNES VAN HARDENBERG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,335,265 Dodington Nov. 30, 1943 2,391,776 Fredendall Dec. 25, 1945 

